The questions you have to ask yourself when composing an opening repertoire:

1. “What are my goals in chess?"
2. “Do I mind if I have to learn a new opening in a year or two? Or would I prefer to learn lines I can play for the rest of my life so I can focus on other aspects of chess?"
3. “Do I want to have just one opening or would I like to be able to confidently play two or three different openings?”

If your goals in chess are relatively modest, (i.e- no higher than 2200), you can play many less conventional openings and get away with it. Once you have loftier goals you may be able to get away with more dubious lines, but you’ll be putting yourself at a handicap. Below are my recommendations for various rating goals.
Your goals are to get to up to 1800:
Pick openings based on enjoyment. You can even play strange stuff like the King’s Gambit as your main opening or the Budapest Gambit. You can play stuff that’s completely unsound because the large majority of sub 2000 players won’t be able to take advantage of your dubious opening choices. Understand that if your goals change, you may need to learn something new according to your new ambitions. If this is only a short term goal, and at some point in the future you’d like to become even stronger, you will be cheating yourself by not immediately learning more serious variations.
Your goals are to get to 2000:
Now you should be playing more solid lines and I’d avoid openings that are simply considered bad such as the Wing Gambit of the Sicilian. However you can feel free to play something like the Grand Prix Attack (even though I think it sucks).
Your goals are to get to 2200:
Believe it or not you still have some leeway here, as quite a few players reach 2200 with complete rubbish opening repertoires. In order to increase your chances I’d recommend learning some solid openings. If you really want you can play the Alekhine’s Defense every game, or the London system for white, though I’d never recommend these to any of my students as a primary opening choice.

The minimum openings I’d recommend for black would be stuff like the Benko Gambit, Scandinavian Defenses (although I think the Scandinavian is a really lame opening) and so forth, while with white you can still get away with the c3 Sicilian, and 3.Nc3, 3.Nd2 or 3. e5 against the French (but no more 3.exd5 as your only choice against the French!).

I don't like Catalan type structures as laid out in Avrukh’s book until one is at the 2200+ level. The positions are too subtle, require too much memorization in some variations to hope for any advantage against a prepared opponent, and I believe players should be learning more classical chess until they are higher rated. I have similar feelings about the English opening. Save these hypermodern approaches for later in your career and don’t start out with them unless your ambitions are modest.
Your goals are to get to 2400 and beyond:
Now things change a lot. Let’s assume you are a player who will play only one opening against each reply by your opponent. Now the number of acceptable lines dwindles quite a bit, especially against 1. d4. For instance, against 1.d4 there are just six common lines played at the top level these days:

1. The Queen's Gambit Declined: The traditional QGD is relatively solid and boring but super hard to crack (although annoying to play against lower rated people), while sharper Ragozin systems with ...Bb4 are becoming more popular lately.

2. The Grunfeld Defense: You have to know a ton but it’s extremely difficult for white to find any advantage.3. The Slav Defense: Ever popular and super-solid. Just be sure you have some sharp ideas against the Exchange Slav against significantly lower rated players4. The Semi-Slav Defense: This is also super-solid, and a bit sharper than the Slav.5. The Nimzo-Indian and the Bogo or the Queen's Indian- These lead to dynamic games and are very theoretically sound.6. The King’s Indian Defense-The riskiest choice of the bunch, but it seems like black’s getting away with playing it.

Of the above, 2, 4, 5 and 6 are the most combative, while 1+3 are more solid (excluding the Ragozin), which can be annoying against lower rated opponents. Lines such as the Queen’s Gambit Accepted, Benoni, Benko Gambit and so forth, are rarely the exclusive opening choice of anyone at the high Grandmaster level.

Against 1. e4 you should probably be playing one of four moves on Move 1:

1. 1...c5. The Sicilian is the sharpest choice as well as being theoretically sound. The safest bet is probably the Najdorf, as white has an extremely hard time finding an edge, but it’s exceptionally sharp. The other cool thing about the Najdorf is that there are so many ways to play it, so if one line bugs you (for instance 6. Be3 e5), you can just switch to another version of the Najdorf (ie 6. Be3 e6, or 6…Ng4). The Classical Sicilian with 5...Nc6 is not as popular but also seems super solid if you know what you’re doing and play the early ….a6, Bd7 stuff (the old main lines with an early Be7 and 0-0 look a little shaky to me). Riskier choices such as the Dragon, Scheveningen, Kan, Sveshnikov etc etc., are also playable as a primary opening up to at least 2400-2500 FIDE. That being said, I’d recommend the Najdorf above all of these lines.
2. 1..e5. This is an extremely solid choice and from a theoretical standpoint White will rarely get more than a tiny edge. The Petroff is a bit dry, so it will be hard to beat lower rated players, so 2….Nc6 is a much smarter practical choice. You can play lots of defenses against the Ruy Lopez….such as 3…a6 followed by an early …b5 or the old main lines with 8….d6 or the Marshall Gambit. 3…Nf6 is exceptionally solid, although I’d be wary to go into the Berlin Wall in every game.
3. 1…c6 is super solid and white has tremendous difficulty finding an edge. Tons of top players in the world play the Caro-Kann regularly with strong results. Even if your opponent knows for sure that you will play 1…c6, there is not that much they can do about it if you're prepared, and if you know your stuff you should get a fine opening position. The only concern with this choice is that I believe it’s more difficult to beat lower rated opposition with the Caro-Kann, but otherwise I think it’s awesome.
4. 1….e6 is also a solid choice. My personal feeling is that the French is a bit riskier than the previous three choices, but it’s probably just a matter of taste. Against 3. Nd2 you have a ton of choices, 3…Nf6, c5 and Be7 which are all fine for black. Against 3. Nc3 you can play the sharp 3…Bb4 or the solid 3…Nf6. Personally I believe that white cannot really count on an advantage against a well prepared French player.

If you have big dreams and don’t want to spend a ton of time learning openings, just pick one of the lines above, learn it deeply, and get ready to play it for a while. Realistically you’ll need to have a second defense against 1.e4/1.d4 down the line. You can either pick from the recommended lines above, or throw in a line that is a little less theoretically sound but still quite playable as a surprise weapon (i.e- the Benoni or the Pirc/Alekhine).

With white it makes sense to play relatively main lines. There’s a reason that 90% of titled players use 2. Nf3 against the Sicilian and the large majority of them follow up with 3. d4. It’s daunting but if you want to be great at chess someday, you should probably do the hard work as early as possible, so that as you grow, you already have a base of knowledge and a rich and cultured knowledge of topical chess openings to help develop your chess understanding. If your coach insists that you play rare sidelines and you have high chess ambitions, tell them “no” and that you’d like to learn some main lines. The rare sidelines may help with short term results but in the long run it’s nice when you play the same openings the top players in the world play. It’s fun to watch Kramnik, Anand or Carlsen play your opening and learn many valuable lessons from how they handle it. Who knows, if you are researching and consistently playing the most viable theoretical lines, they may be looking at your games one day too.

My next article will focus on why I think proper opening study is unfairly criticized and is actually an underrated method of improving and enriching one’s chess understanding for players of all levels.